There are mixed reports on this. As soon as we advise that Product Keys are supposed to be language-specific, someone will report they have reinstalled another language in the same version and activated with their Product Key. This may be due to variations in OEM factory installs, or certain groupings of languages which are included with retail Product Key.
What I would do is save externally a Windows 7 backup image of your current install, and back up your files.
Try running a Repair Install in the language of your choice, then try to activate with the Product Key. Repair Install
If it refuses to run the Repair Install (in-place Upgrade) then do a clean reinstall, see if that will activate with Prod Key. If not, speak with an MS activation agent to see if they will waive it through.
If not, then reimage your old install back onto HD.